The present invention relates to magnetic discs used in computer disc drives, and, more particularly, to the use of texturing of a thin film magnetic disc surface and to processing of lubricant used on the thin film magnetic disc for reduced stiction associated with stop times of the disc drive.
Computer disc drives commonly use thin film media to store information in a high-density, high-reliability format. The magnetic layer in thin film media is typically provided by a cobalt alloy film. The cobalt alloy film may be applied at a thickness of around 500 Angstroms over a supporting substrate. The substrate may be nickel-phosphorous plated aluminum which may be coated with a chromium underlayer for the magnetic media.
While thin film media offer important advantages toward higher storage density, the cobalt alloy films are significantly less durable than films composed of the magnetic and alumina particles used in particulate media. To enhance the durability of the disc, a protective layer of a very hard material is applied over the cobalt alloy film. A typical protective layer is an overcoat of sputtered amorphous carbon about 150 Angstroms thick. The amorphous carbon overcoat structure can be thought of as a hybrid between diamond and graphite. Other materials which have been used for overcoats include sputtered ceramic zirconium oxide and amorphous films of silicon dioxide.
The overcoat surface is usually lubricated to further reduce wear of the disc due to contact with the magnetic head assembly. The lubricant is typically applied evenly over the disc in a molecularly thin film having a thickness from 10 to 50 Angstroms. Thicker films tend to be spun off by centrifugal forces. Slider-disc interaction, air shear and evaporation may also affect the amount of lubricant on a disc.
Disc surfaces have been textured to affect the tribology between the slider and the disc. A surface roughness of 50 to 350 Angstroms peak-to-valley is typical. The texturing can be provided by texturing of any of the disc layers. For instance, the overcoat layer may be applied over a smooth surfaced magnetic layer, with the overcoat layer being mechanically textured after it has been applied. Alternatively, the substrate may be mechanically textured prior to application of any of the covering layers. Each of the underlayer, magnetic layer and overcoat are typically applied by sputtering to provide a very even thickness. Accordingly, when the underlayer, magnetic layer and overcoat are applied over a textured substrate, the texturing is transmitted through each of these layers, and the resultant surface of the overcoat layer retains most of the texturing of the substrate.
Magnetic discs in a computer disc drive which use an air bearing slider have two discrete zones which may be defined based on how the slider of the magnetic head assembly travels over the disc surface. A xe2x80x9clanding zonexe2x80x9d is the zone where the slider containing the read/write transducer lands, rests while the disc drive is off, and takes off from when the disc guide is started up. A xe2x80x9cdata zonexe2x80x9d is the zone where the slider flies over the disc and stores magnetic data. Lubricant is generally applied both to the landing zone and the data zone on the magnetic disc.
When the power is switched on in a disc drive, enough force has to be applied to the slider to overcome the static friction or xe2x80x9cstictionxe2x80x9d force on the slider. During the relatively low speeds encountered during takeoff and landing, the slider maintains almost constant contact with the disc. Lubricant over the landing zone is important for its contribution toward the stiction force and to minimize wear and drag force during takeoff and landing.
As the speed of the disc increases, the slider bears off the air in contact with the disc surface such that the magnetic head assembly becomes airborne. During use of the disc drive, the magnetic head assembly is designed to fly over the disc surface without contacting the disc. However, the magnetic head assembly occasionally contacts the disc during use of the disc drive. These in-flight contacts between the slider and the media occur infrequently, but at high speeds. Most of these contacts are caused by collision of the slider with media asperities, third bodies such as corrosion products, or other contaminant particles. The lubricant on the data zone minimizes wear and/damage to the disc due to these occasional contacts between the magnetic head assembly and the disc.
Perfluoropolyethers (PFPEs) are currently the lubricant of choice for thin film recording media. PFPEs are long chain polymers composed of repeat units of small perfluorinated aliphatic oxides such as perfluoroethylene oxide or perfluoropropylene oxide. As a class of compounds, PFPEs provide excellent lubricity, a wide liquid-phase temperature range, low vapor pressure, small temperature dependency of viscosity, high thermal stability, and low chemical reactivity. PFPEs also exhibit low surface tension, resistance to oxidation at high temperature, low toxicity, and moderately high solubility for oxygen. Several different PFPE polymers are available commercially, such as Fomblin Z (random copolymer of CF2CF2O and CF2O units) and Y (random copolymer of CF(CF3)CF2O and CF2O) including Z-DOL and AM 2001 from Montedison, Demnum (a homopolymer of CF2CF2CF2O) from Daikin, and Krytox (homopolymer of CF(CF3)CF2O). Fomblin Z and Y are prepared by photo-oxidation of tetrafluoroethylene and hexafluoropropylene, respectively and are random copolymers of indicated units. Krytox and Demnum are synthesized via base catalyzed polymerization of perfluoropropylene oxide and trimethylene oxide, respectively. See U.S. Pat. No. 3,242,218 and U.S. Pat. No. 3,665,041. Commercially available PFPEs generally have a very broad molecular weight distribution. Lubricants are available in a number of average molecular weights, such as 2000 a.m.u. (Z-DOL 2000 for instance), 6000 a.m.u. (Fomblin Z), etc.
In some cases, some or all of the PFPE lubricant molecules have been bonded to the surface of the disc. Bonding helps to reduce lubricant which may be lost due to spin-off, evaporation, or chemical displacement. It has also been theorized that bonding of the lubricant helps to lower stiction forces.
Different strategies have been employed for bonding the lubricant molecules to a disc surface to enhance lubricant performance. For instance, the molecules may be chemically bonded to the disc surface. The PFPE lubricant molecules may be terminated by reactive, functional end-groups, such as hydroxyl, carboxyl, or piperonyl. The end groups chemically react with the amorphous carbon overcoat to bond the lubricant to the disc surface. Similarly, ionically polar end groups may be used to bond the lubricant to the disc surface. Lubricant bonding schemes may also involve thermal treatment or exposure to ultra violet light.
With thin film magnetic media, decreasing head to media spacing is critical for higher storage densities. Present flying altitudes of magnetic head assemblies over the disc surface are usually in the 100-500 Angstrom range. Still lower flying altitudes are anticipated in the future. As flying altitudes are decreased, the tribology between the slider and disc surface becomes more and more important. While the tribology between a slider and a disc is a function of the properties of the substrate and all the deposited layers, the overcoat and the lubricant are of primary importance. The slider structure also greatly affects the tribology, and sliders are usually formed of fairly hard ceramics such as Mnxe2x80x94Zn ferrite, calcium titanate (CaTiO3) and Al2O3xe2x80x94TiC.
Stiction is one of the most important and complex tribological phenomenon for disc drives. High stiction forces can lead the disc drive to fail, as the motor may not be strong enough to overcome the initial stiction. The amount of stiction is a function of storage time as well as the normal force between the slider and the disc. The time dependency associated with the storage time is accredited to lubricant migration toward points of contact between the slider and the disc surface, resulting in meniscus forces which increase slowly over time. The time dependency of stiction may also be the result of increased elastic deformation of contacting asperities between the slider and disc surface, as well as the result of slow diffusion of ambient species, mainly water, into the lubricated junctions followed by displacement of lubricant from the meniscus.
The present invention is a method and apparatus to dramatically reduce stiction over a thin film disc. A light abrasive texture applied over the data zone, while a surface texture of a second roughness applied over the landing zone. A lubricant is applied over both the landing zone and the data zone. The lubricant is preferably a high molecular weight PFPE lubricant which has been supercritically fractionated in carbon dioxide to exclude the fractions of low molecular weight and to exclude the fractions of very high molecular weight.